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"Extremists continue to focus on tourist locations, transportation hubs, markets/shopping malls, and local government facilities as viable targets," the department said. "In addition, hotels, clubs, restaurants, places of worship, parks, high-profile events, educational institutions, airports, and other soft targets remain priority locations for possible attacks. U.S. citizens should exercise additional vigilance in these and similar locations, in particular during the upcoming summer travel season when large crowds may be common."

Attacks claimed by groups like the Islamic State and al-Qa’ida have surged across Europe in recent months, leaving death and destruction in their wake.

On March 22, a man wielding a knife drove an SUV into pedestrians near Parliament. Though the attack lasted only 82 seconds, the incident left five people dead and 20 injured. Islamic extremist Khalid Masood was fatally shot at the scene.

Last month, an explosion tore through a subway train in St. Petersburg, Russia, killing 16 people and wounding more than 40. Perpetrator Akbarzhon Jalilov, a Russian citizen, was carrying out orders from al Qaeda, according to militant Islamist organization Imam Shamil Battalion last week.

"Terrorists persist in employing a variety of tactics, including firearms, explosives, using vehicles as ramming devices, and sharp-edged weapons that are difficult to detect prior to an attack," the State Department warned.